The inborn errors of metabolism currently under investigation involve those of lysine and branched-chain amino acid degradation, and of Lesch-Nyhan disease. Most studies are conducted on skin fibroblasts grown in tissue culture. BIBLIOGRAPHIC REFERENCES: Francke, U., Felsenstein, J., Gartler, S. M., Migeon, B.R., Dancis, J., Seegmiller, J.E., Bakay, B. and Nyhan, W.L.: The occurrence of new mutants in the X-linked recessive Lesch-Nyhan Disease. Am. J. Human Genet., 28:123, 1976. Holland, M.J.C., DiLorenzo, A.M., Dancis, J., Balis, M.E., Yu, T.F., and Cox, R.P.: Hypoxanthine phosphoribosyltransferase activity in intact fibroblasts from patients with X-linked hyperuricemia. J. clin. Invest., 57:1600, 1976.